Yes, I did find a cause and a solution, but it took a lot of time, not to mention persistence. I was fortunate in that I could use on a separate system. This allowed me to do a gradual search and replace to establish which particular Style was causing the problem.

I'll quote what I hope are relevant parts of the exchange with MacCap.

(MCap) After much troubleshooting, we have found that the issue has to do with the use of the font family "Arial Unicode MS." This font family is set over 70 times throughout the stylesheet. Once we removed all references to this font, the output shows correct in Edge. We did not see any difference in the output's font styling when all of these references were removed.

In my reply next, please note that I was working on the "PRINT" section of the .css

(Me) I appear to have narrowed it down to 6 changes of print Styles "Font Family" where it now just has "Arial". These are:

bbodycaptiondivh2 -- NoNewPage Madcap|slideshowCaption

[GB - Finally!] I changed the Font Family of the "b" Style in the print section of the DuxHelp.css to Arial, recompiled, and the blank pages are no more in Edge.

I still have not established if this is a Flare or Edge problem, but suspect the latter.